Closed loop type fiber optic gyroscope for measuring absolute rotation by delta serrodyne wave phase modulation

Abstract

A fiber optic gyro can overcome the defects encountered with conventional fiber optic gyros of phase-modulation method, closed-loop system with serrodyne modulation and digital modulation. A reference phase difference .DELTA..beta. and a ramp phase difference .sigma. are generated in an interference light intensity signal I by use of a triangular waveform, i.e., delta serrodyne waveform signal. The reference phase difference .DELTA..beta. is changed to constant values .DELTA..beta..sub.A and .DELTA..beta..sub.B whose absolute values are the same and whose signs are different at every times T.sub.A and T.sub.B. A phase x of the interference light intensity signal I becomes equal to x=.DELTA..theta.+.sigma.+.DELTA..beta.. The ramp phase difference .sigma. is controlled so as to satisfy .DELTA..theta.+.sigma.=0. Accordingly, at the stable point of the control loop, a Sagnac phase difference .DELTA..theta. is equal to the ramp phase difference .sigma.. Further, a phase x of the interference light intensity signal I does not contain the Sagnac phase difference .DELTA..theta., and hence x=.DELTA..beta.. Also, a fiber optic gyro can eliminate a bias, in particular, optically-generated bias. In a fiber optic gyro of phase-modulation method or delta serrodyne system, a phase controller is supplied with a phase control voltage signal V.sub.S of period T and a square wave signal V.sub.2 of period T for correcting an optical bias caused by an amplitude modulation generated by the phase control voltage signal V.sub.S in the added form. The square wave signal V.sub.2 has a constant magnitude H and is inverted in polarity at points in which the phase control voltage signal V.sub.S is maximized or minimized. The magnitude H and the polarity of the square wave signal V.sub.2 are selected such that an optical bias is minimized.

Claims

1. A fiber optic gyro comprising a light source; an optical fiber loop; means for producing a control voltage input signal having a triangular wave form including an up slope linear portion corresponding to a first time T.sub.A and a down slope linear portion corresponding to a second time T.sub.B, said input signal having a constant period T where T=T.sub.A +T.sub.B; a first propagating beam and a second propagating beam propagating along said optical fiber loop in opposite directions; a phase controller for receiving said control voltage input signal and adjusting a phase differential between said first and second propagating beams according to said control voltage input signal; a light receiving device for detecting an interference between said first and second propagating beam, converting said interference into a interference light intensity signal I and outputting said signal; said phase controller generating a reference phase difference.DELTA..beta. and a ramp phase difference.DELTA..sigma. in said interference light intensity signal I, said reference phase difference.DELTA..beta. having a first reference phase difference portion.DELTA..beta..sub.A corresponding to first time T.sub.A and a second reference phase difference portion.DELTA..beta..sub.B corresponding to second time T.sub.B, said first and second reference phase difference portions having opposite polarities and equal absolute values, means for receiving said light intensity signal I and detecting from said signal a difference between a Sagnac phase difference.DELTA..THETA. and said ramp phase difference.DELTA..sigma.; said ramp phase difference.DELTA..sigma. having a first inclination corresponding to a phase difference.DELTA..beta..sub.A +.sigma. during said first time T.sub.A and a second inclination corresponding to a phase difference.DELTA..beta..sub.B during said second time T.sub.B, one of said first and second inclinations having a positive value and the other having a negative value, thereby producing a delta serrodyne wave signal having a waveform that inclines at every time T.sub.A and T.sub.B; means for controlling said times T.sub.A and T.sub.B so as to thereby retain a mean value of said triangular wave within a predetermined amplitude, whereby a rotation rate.OMEGA. is obtained from the Sagnac phase difference.DELTA..THETA. generated in said interference light intensity signal I when said optical fiber is rotated around a central axis of said loop at said rotation rate.OMEGA..

2. A fiber optic gyro as claimed in claim 1, wherein said reference phase difference.DELTA..beta. becomes.DELTA..beta..sub.A =-(2n-1).pi./2 during said first time T.sub.A and.DELTA..beta..sub.B =+(2n-1).pi./2 during said second time T.sub.B where n is a positive integer.

4. A fiber optic gyro as claimed in claim 1, wherein when a positive time in one period of said delta serrodyne waveform signal is T.sub.+ and a negative time is T.sub.-, durations of said first time T.sub.A and said second time T.sub.B are adjusted based on a difference.SIGMA.T.sub.+ -.SIGMA.T.sub.- or a difference.SIGMA.(T.sub.+ -T.sub.-).

5. A fiber optic gyro as claimed in claim 4, wherein an input rotation rate.OMEGA. or a rotation angle is computed based on a difference.SIGMA.T.sub.+ -.SIGMA.T.sub.- or a difference.SIGMA.(T.sub.+ -T.sub.-) when a positive time in one period T of said delta serrodyne waveform signal is T.sub.+ and a negative time is T.sub.-.

6. A fiber optic gyro as claimed in claim 5, wherein when said positive time T.sub.+ and said negative time T.sub.- are counted by a pulse of a predetermined period and pulse numbers are respectively set to N.sub.+ and N.sub.- an input rotation rate and a rotation angle are computed based on a difference.SIGMA.N.sub.+ -.SIGMA.N.sub.- or difference.SIGMA.(N.sub.+ -N.sub.-).

7. A fiber optic gyro as claimed in claim 1, wherein the slope of said control voltage supplied to said phase controller corresponds to a sum of a constant reference voltage signal V* corresponding to said reference phase difference and a ramp voltage signal V.sub.R corresponding to said ramp phase difference.sigma. and said ramp voltage signal V.sub.R is generated by integrating a voltage signal corresponding to a difference signal.DELTA.I between a value I.sub.A of said interference light intensity in said first time T.sub.A and a value I.sub.B of said interference light intensity I in said second time T.sub.B.

8. A fiber optic gyro according to claim 1, further comprising:

a signal processing unit for receiving said interference light intensity signal I outputted from said light-receiving device and generating a voltage signal V.sub.0 corresponding to a difference signal.DELTA.I=I.sub.A -I.sub.B,

an integrator for receiving and integrating said voltage signal V.sub.0; and

a delta serrodyne unit for receiving an output signal V.sub.R of said integrator and generating said delta serrodyne waveform signal.

9. A fiber optic gyro as claimed in claim 8, wherein said signal processing unit includes a DC cancel circuit for canceling a DC component from said interference light intensity signal I to generate an alternating signal which alternately changes to.+-..DELTA.I/2 at every times T.sub.A and T.sub.B, an AC amplifier for AC-amplifying an output signal from said DC cancel circuit and a demodulator for obtaining said DC voltage signal V.sub.0 from an output signal of said AC amplifier.

10. A fiber optic gyro as claimed in claim 8 or 9, wherein said delta serrodyne unit includes an adder for adding a reference voltage signal V* whose sign alternately changes to positive or negative at every times T.sub.A, T.sub.B and said ramp voltage signal V.sub.R outputted from said integrator and a delta serrodyne integrator for integrating an output signal from said adder.

11. A fiber optic gyro as claimed in any one of claims 7, 8, or 9, further comprising a reference phase control unit for generating said reference voltage signal V* by use of a voltage signal corresponding to a mean value I.sub.0 between said interference light intensity signal I.sub.A in said first time T.sub.A and said interference light intensity signal I.sub.B in said second time T.sub.B.

12. A fiber optic gyro comprising a light source, an optical fiber loop, a phase controller for changing a phase differential between said first propagating beam and second propagating beam propagating along said optical fiber loop in opposite directions and a light receiving device for detecting interference light of said first propagating beam and said second propagating beam in which a rotation rate.OMEGA. is obtained from the Sagnac phase difference.DELTA..THETA. generated in an interference light intensity signal I when said optical fiber loop is rotated around a central axis of said loop at said rotation rate.OMEGA., wherein said phase controller is supplied with a phase control voltage signal V.sub.S of period T where T=T.sub.a +T.sub.b for controlling a phase between said first propagating beam and said second propagating beam and a square wave signal V.sub.2 of period T in the added form; said square wave signal V.sub.2 has a constant magnitude H and is inverted in polarity at time points in which said phase control voltage signal V.sub.S is maximized and minimized; said magnitude H and said polarity of said square wave signal V.sub.2 are selected in such a manner that an optical bias is minimized; and means for controlling said times T.sub.a and T.sub.b to thereby retain a mean value of said control voltage signal V.sub.S within a predetermined amplitude.

13. A fiber optic gyro as claimed in claim 12, wherein said magnitude and said polarity of said square wave signal are selected such that a bias caused by an amplitude modulation generated in accompaniment with a phase modulation is minimized.

14. A fiber optic gyro as claimed in claim 12 or 13, wherein said interference light intensity signal I outputted from said light receiving device is demodulated by a demodulation signal having the same frequency as that of said phase control voltage signal V.sub.S and an operation for switching polarities of said demodulation signal is synchronized with points at which said phase control voltage signal V.sub.S is maximized or minimized with a constant phase difference.

15. A fiber optic gyro as claimed in claim 12 or 13, wherein said phase controller generates a reference phase difference.DELTA..beta. and a ramp phase difference.sigma. in said interference light intensity signal I, said reference phase difference.DELTA..beta. has a constant period T, said reference phase difference.DELTA..beta. becomes first and second phase differences.DELTA..beta..sub.A,.DELTA..beta..sub.B during first and second times T.sub.A, T.sub.B of one period T, said first and second reference phase differences.DELTA..beta..sub.A,.DELTA..beta..sub.B are opposite in sign but equal to each other in absolute value, said ramp phase difference.sigma. is controlled so as to cancel said Sagnac phase difference.DELTA..theta. and fed back in phase to said propagating beam, said phase control voltage V.sub.S supplied to said phase controller has a first inclination corresponding to a phase difference.DELTA..beta..sub.A +.sigma. during said first time T.sub.A and a second inclination corresponding to a phase difference.DELTA..beta..sub.B +.sigma. during said second time T.sub.B and one of said first and second times T.sub.A, T.sub.B becomes negative and the other becomes positive, thereby presenting a delta serrodyne waveform signal of triangular wave which inclines at every first and second times T.sub.A, T.sub.B.

16. A fiber optic gyro as claimed in claim 15, wherein said reference phase difference.DELTA..beta. becomes.DELTA..beta.=-(2n-1).pi./2 during said first time T.sub.A and.DELTA..beta..sub.B =+(2n-1).pi./2 during said second time T.sub.B where n is a positive integer.

17. A fiber optic gyro as claimed in claim 15, wherein a sum of said first time T.sub.A and said second time T.sub.B composing one period of said delta serrodyne waveform signal is constant T=T.sub.A +T.sub.B and durations of said first time T.sub.A and said second time T.sub.B are adjusted in such a manner that a peak value of said delta serrodyne waveform signal does not exceed a predetermined allowable value.

18. A fiber optic gyro according to claim 15, further comprising:

a signal processing unit for receiving said interference light intensity signal I outputted from said light-receiving device and generating a voltage signal V.sub.0 corresponding to a difference signal.DELTA.I=I.sub.A -I.sub.B,

an integrator for receiving and integrating said voltage signal V.sub.0; and

a delta serrodyne unit for receiving an output signal V.sub.R of said integrator and generating said delta serrodyne waveform signal.

19. A fiber optic gyro as claimed in claim 18, wherein said signal processing unit includes a DC cancel circuit for canceling a DC component from said interference light intensity signal I to generate an alternating signal which alternately changes to.+-..DELTA.I/2 at every times T.sub.A and T.sub.B, an AC amplifier for AC-amplifying an output signal from said DC cancel circuit and a demodulator for obtaining said DC voltage signal V.sub.0 from an output signal of said AC amplifier.